Critical Conditions Regulating the Gelation in Macroionic Cluster Solutions

The critical gelation conditions observed in dilute aqueous solutions of multiple nanoscale uranyl peroxide molecular clusters are reported, in the presence of multivalent cations. This gelation is dominantly driven by counterion-mediated attraction. The gelation areas in the corresponding phase diagrams all appear in similar locations, with a characteristic triangle shape outlining three critical boundary conditions, corresponding to the critical cluster concentration, cation/cluster ratio, and the degree of counterion association with increasing cluster concentration. These interesting phrasal observations reveal general conditions for gelation driven by electrostatic interactions in hydrophilic macroionic solutions.

Insight into the Structural Ambiguity of Actinide(IV) Oxalate Sheet Structures: A Case for Alternate Coordination Geometries

Invited for the cover of this issue is the group of Amy Hixon at the University of Notre Dame. The image depicts the newly identified structure of a PuIV oxalate sheet compared to the historically assumed structure. Read the full text of the article at 10.1002/chem.202301164.

Insight into the Structural Ambiguity of Actinide(IV) Oxalate Sheet Structures: A Case for Alternate Coordination Geometries

Plutonium(IV) oxalate hexahydrate (Pu(C2 O4 )2  ⋅ 6 H2 O; PuOx) is an important intermediate in the recovery of plutonium from used nuclear fuel. Its formation by precipitation is well studied, yet its crystal structure remains unknown. Instead, the crystal structure of PuOx is assumed to be isostructural with neptunium(IV) oxalate hexahydrate (Np(C2 O4 )2  ⋅ 6 H2 O; NpOx) and uranium(IV) oxalate hexahydrate (U(C2 O4 )2  ⋅ 6 H2 O; UOx) despite the high degree of unresolved disorder that exists when determining water positions in the crystal structures of the latter two compounds. Such assumptions regarding the isostructural behavior of the actinide elements have been used to predict the structure of PuOx for use in a wide range of studies. Herein, we report the first crystal structures for PuOx and Th(C2 O4 )2  ⋅ 6 H2 O (ThOx). These data, along with new characterization of UOx and NpOx, have resulted in the full determination of the structures and resolution of the disorder around the water molecules. Specifically, we have identified the coordination of two water molecules with each metal center, which necessitates a change in oxalate coordination mode from axial to equatorial that has not been reported in the literature. The results of this work exemplify the need to revisit previous assumptions regarding fundamental actinide chemistry, which are heavily relied upon within the current nuclear field.

Solubility and Thermodynamic Investigation of Meta-Autunite Group Uranyl Arsenate Solids with Monovalent Cations Na and K

We investigated the aqueous solubility and thermodynamic properties of two meta-autunite group uranyl arsenate solids (UAs). The measured solubility products (log Ksp) obtained in dissolution and precipitation experiments at equilibrium pH 2 and 3 for NaUAs and KUAs ranged from -23.50 to -22.96 and -23.87 to -23.38, respectively. The secondary phases (UO2)(H2AsO4)2(H2O)(s) and trögerite, (UO2)3(AsO4)2·12H2O(s), were identified by powder X-ray diffraction in the reacted solids of KUA precipitation experiments (pH 2) and NaUAs dissolution and precipitation experiments (pH 3), respectively. The identification of these secondary phases in reacted solids suggest that H3O+ co-occurring with Na or K in the interlayer region can influence the solubilities of uranyl arsenate solids. The standard-state enthalpy of formation from the elements (ΔHf-el) of NaUAs is -3025 ± 22 kJ mol-1 and for KUAs is -3000 ± 28 kJ mol-1 derived from measurements by drop solution calorimetry, consistent with values reported in other studies for uranyl phosphate solids. This work provides novel thermodynamic information for reactive transport models to interpret and predict the influence of uranyl arsenate solids on soluble concentrations of U and As in contaminated waters affected by mining legacy and other anthropogenic activities.

Targeting Diverse Bridging Motifs within Actinide Borosulfates and Establishing an Unconventional Structural Hierarchy

The first actinide borosulfates, (UO₂)[B(SO₄)₂(SO₃OH)] (TSUBOS-1) and (UO₂)₂[B₂O(SO₄)₃(SO₃OH)₂] (TSUBOB-1), were synthesized solvothermally in oleum using UO₃. The classical borosulfate crystal structure of TSUBOS-1 is partially consistent with an established conventional hierarchy. Uranyl pentagonal bipyramids limit the anionic network linkages and isolate sulfate tetrahedra within the anionic network. Therefore, the classical one-dimensional chain established in the hierarchy does not fully describe the structure. The structure of TSUBOB-1 is the first actinide borosulfate that contains an unconventional borate-to-borate bridging mode (denoted B-O-B) and a zero-dimensional oxoanionic unit consisting of one sulfate tetrahedron that shares vertices with two B-O-B bridged borate tetrahedra that each share a vertex with two sulfate tetrahedra. As this structure departs from the existing structural hierarchy, a modified approach for understanding the unconventional borosulfate substructure and dimensionality is proposed, together with a new graphical notation. In the course of our synthesis experiments, a novel uranyl disulfate compound (UO₂)₂[(S₂O₇)(SO₃OH)₂] (TSUDS) was isolated and characterized. The structure of TSUDS is a framework consisting of uranyl pentagonal bipyramids and sulfate tetrahedra. Each uranyl pentagonal bipyramid is surrounded by five sulfate tetrahedra, two of which share a vertex creating a disulfate with a S-O-S bridging mode. The uranyl bipyramids are linked to one another via the singular sulfate or disulfate groups.

Radiation-Induced Solid-State Transformations of Uranyl Peroxides

Single-crystal X-ray diffraction studies of pristine and γ-irradiated Ca₂[UO₂(O₂)₃]·9H₂O reveal site-specific atomic-scale changes during the solid-state progression from a crystalline to X-ray amorphous state with increasing dose. Following γ-irradiation to 1, 1.5, and 2 MGy, the peroxide group not bonded to Ca²⁺ is progressively replaced by two hydroxyl groups separated by 2.7 Å (with minor changes in the unit cell), whereas the peroxide groups bonded to Ca²⁺ cations are largely unaffected by irradiation prior to amorphization, which occurs by a dose of 3 MGy. The conversion of peroxide to hydroxyl occurs through interaction of neighboring lattice H₂O molecules and ionization of the peroxide O-O bond, which produces two hydroxyls, and allows isolation of the important monomer building block, UO₂(O₂)₂(OH)₂^4-, that is ubiquitous in uranyl capsule polyoxometalates. Steric crowding in the equatorial plane of the uranyl ion develops and promotes transformation to an amorphous phase. In contrast, γ-irradiation of solid Li₄[(UO₂)(O₂)₃]·10H₂O results in a solid-state transformation to a well-crystallized peroxide-free uranyl oxyhydrate containing sheets of equatorial edge and vertex-sharing uranyl pentagonal bipyramids with likely Li and H₂O in interlayer positions. The irradiation products of these two uranyl triperoxide monomers are compared via X-ray diffraction (single-crystal and powder) and Raman spectroscopy, with a focus on the influence of the Li⁺ and Ca²⁺ countercations. Highly hydratable and mobile Li⁺ yields to uranyl hydrolysis reactions, while Ca²⁺ provides lattice rigidity, allowing observation of the first steps of radiation-promoted transformation of uranyl triperoxide.

Standalone 2-D Nanosheets and the Consequent Hydrogel and Coacervate Phases Formed by 2.5 nm Spherical U60 Molecular Clusters in Dilute Aqueous Solution

Unexpected hydrogel and coacervate are observed for dilute (1 mM) uranyl peroxide molecular cluster (Li₆₈K₁₂(OH)₂₀[UO₂(O₂)(OH)]₆₀, U₆₀) solution in the presence of di- or trivalent salts. We report the mechanism as the formation of anisotropic two-dimensional (2-D) single-layer nanosheets, driven by counterion-mediated attraction due to the size disparity between U₆₀ and small counterions. With weak monovalent cations, the nanosheets are bendable, resulting in hollow, spherical blackberry-type supramolecular assemblies in a homogeneous solution. With extra strong divalent or trivalent cations, the tough, free-standing sheets lead to gelation at ∼1 mM U₆₀. These stiff nanosheets are difficult to bend into spherical blackberry-type structures; instead, they stay in solution and form hydrogel based on their significant excluded volumes. At higher ionic strength, the large, thin filmlike nanosheet structures stack together more compactly and consequently lead to the transition from gel phase to a coacervate phase, another surprise since it was formed without the presence of bulky polycations.

Unusual Metal-Organic Framework Topology and Radiation Resistance through Neptunyl Coordination Chemistry

A Np(V) neptunyl metal-organic framework (MOF) with rod-shaped secondary building units was synthesized, characterized, and irradiated with γ rays. Single-crystal X-ray diffraction data revealed an anionic framework containing infinite helical chains of actinyl-actinyl interaction (AAI)-connected neptunyl ions linked together through tetratopic tetrahedral organic ligands (NSM). NSM exhibits an unprecedented net, demonstrating that AAIs may be exploited to give new MOFs and new topologies. To probe its radiation stability, we undertook the first irradiation study of a transuranic MOF and its organic linker building block using high doses of γ rays. Diffraction and spectroscopic data demonstrated that the radiation resistance of NSM is greater than that of its linker building block alone. Approximately 6 MGy of irradiation begins to induce notable changes in the long- and short-range order of the framework, whereas 3 MGy of irradiation induces total X-ray amorphization and changes in the local vibrational bands of the linker building block.

Ionothermal Synthesis of Uranyl Vanadate Nanoshell Heteropolyoxometalates

Two uranyl vanadate heteropolyoxometalates (h-POMs) have been synthesized by ionothermal methods using the ionic liquid 1-ethyl-3-methylimidazolium diethyl phosphate (EMIm-Et₂PO₄). The hybrid actinide-transition metal shell structures have cores of (UO₂)₈(V₆O₂₂) and (UO₂)₆(V₃O₁₂), which we designate as {U₈V₆} and {U₆V₃}, respectively. The diethyl phosphate anions of the ionic liquids in some cases terminate the core structures to form actinyl oxide clusters, and in other cases the diethyl phosphate oxyanions link these cluster cores into extended structures. Three compounds exist for the {U₈V₆} cluster core: {U₈V₆}-monomer, {U₈V₆}-dimer, and {U₈V₆}-chain. Tungsten atoms can partially substitute for vanadium in the {U₆V₃} cluster, which results in a chain-based structure designated as {U₆V₃}-W. Each of these compounds contains charge-balancing EMIm cations from the ionic liquid. These compounds were characterized crystallographically, spectroscopically, and by mass spectrometry.

Reactivity, Formation, and Solubility of Polyoxometalates Probed by Calorimetry

Room temperature calorimetry methods were developed to describe the energy landscapes of six polyoxometalates (POMs), Li-U24, Li-U28, K-U28, Li/K-U60, Mo132, and Mo154, in terms of three components: enthalpy of dissolution (ΔHdiss), enthalpy of formation of aqueous POMs (ΔHf,(aq)), and enthalpy of formation of POM crystals (ΔHf,(c)). ΔHdiss is controlled by a combination of cation solvation enthalpy and the favorability of cation interactions with binding sites on the POM. In the case of the four uranyl peroxide POMs studied, clusters with hydroxide bridges have lower ΔHf,(aq) and are more stable than those containing only peroxide bridges. In general for POMs, the combination of calorimetric results and synthetic observations suggest that spherical topologies may be more stable than wheel-like clusters, and ΔHf,(aq) can be accurately estimated using only ΔHf,(c) values owing to the dominance of the clusters in determining the energetics of POM crystals.

Inhomogeneous Distribution of Cationic Surfactants around Anionic Molecular Clusters

An interesting phenomenon is reported when uranyl peroxide nanoclusters U₆₀ (Li_48+m K₁₂ (OH)_m [UO₂ (O₂ )(OH)]₆₀ (H₂ O)_n , m≈20 and n≈310) interact with a small number of cationic surfactant molecules. Cationic surfactant molecules do not distribute evenly around the U₆₀ clusters during the interaction as expected. Instead, a small fraction of U₆₀ clusters attract almost all the surfactant molecules, leading to the self-assembly into supramolecular structures by using surfactant-U₆₀ complexes as building locks, and later further aggregate and precipitate based on hydrophobic interaction, whereas the rest of the clusters remained unbounded soluble macroions in bulk dispersion. This phenomenon nicely demonstrates a unique feature of macroion solutions. Considering that Debye-Hückel approximation is no longer valid in such solutions, the competition between the local electrostatic interaction and hydrophobic interaction becomes important to regulate the solution behaviors of macroions.

Stability of Solid Uranyl Peroxides under Irradiation

The effects of radiation on a variety of uranyl peroxide compounds were examined using γ-rays and 5 MeV He ions, the latter to simulate α-particles. The studied materials were studtite, [(UO₂)(O₂)(H₂O)₂](H₂O)₂, the salt of the U₆₀ uranyl peroxide cage cluster, Li₄₄K₁₆[(UO₂)(O₂)(OH)]₆₀·255H₂O, the salt of U₆₀Ox₃₀ uranyl peroxide oxalate cage cluster, Li₁₂K₄₈[{(UO₂)(O₂)}₆₀(C₂O₄)₃₀]·nH₂O, and the salt of the U₂₄Pp₁₂ (Pp = pyrophosphate) uranyl peroxide pyrophosphate cage cluster, Li₂₄Na₂₄[(UO₂)₂₄(O₂)₂₄(P₂O₇)₁₂]·120H₂O. Irradiated powders were characterized using powder X-ray diffraction, Raman spectroscopy, infrared spectroscopy, X-ray photoelectron spectroscopy, and UV-vis spectroscopy. A weakening of the uranyl bonds of U₆₀ was found while studtite, U₆₀Ox₃₀, and U₂₄Pp₁₂ were relatively stable to γ-irradiation. Studtite and U₆₀ are the most affected by α-irradiation forming an amorphous uranyl peroxide as characterized by Raman spectroscopy and powder X-ray diffraction while U₆₀Ox₃₀ and U₂₄Pp₁₂ show minor signs of the formation of an amorphous uranyl peroxide.

Hybrid Uranyl-Phosphonate Coordination Nanocage

We report herein a general synthetic approach for designing uranyl coordination cages. Compounds 1 and 2 are constructed through a temperature-dependent and solvent-driven self-assembly. In both cases, the synthetic strategy involves in situ phosphonate ligand condensation into a flexible pyrophosphonate ligand. This pyrophosphonate ligand formation is essential for the introduction of curvature into these compounds. In the presence of PF₆^- ions that are derived from hydrofluoric acid, a macrocyclic uranyl-phosphonate discrete compound, 1, whose cavity contains PF₆^- ions, hydronium ions, and water molecules, is obtained. When Cs⁺ cations are used in the synthesis, a remarkable uranyl coordination nanocage, 2, resulted. The macrocycle (1) is approximately 10.9 × 10.9 Ų in diameter while the nanocage (2) is approximately 15.0 × 11.3 Ų in diameter, as measured from the outer oxygen atoms of the uranyl centers. Both compounds are constructed from a UO₂²⁺ moiety, coordinated by an additional four oxygen atoms from the phosphonate group to form pentagonal bipyramidal geometry. All the compounds fluoresce at room temperature, showing characteristic vibronically coupled charge-transfer based emission.

In situ Raman spectroscopy of uranyl peroxide nanoscale cage clusters under hydrothermal conditions

The behaviours of two uranyl peroxide nanoclusters in water heated to 180 °C were examined by in situ Raman spectroscopy.

Mixed-Valent Cyanoplatinates Featuring Neptunyl-Neptunyl Cation-Cation Interactions

The tetracyanoplatinate ligand was employed in synthesizing the first neptunyl cyanoplatinate complexes. Results indicate in situ oxidation of Pt(II) by Np(V/VI) to form mixed-valent Pt-Pt stacked columnar chains linked by cation-cation interaction induced chains of Np(V) polyhedra into a two-dimensional sheet structure. The Pt-Pt stacking distances of 3.04-3.05 Å are the longest reported columnar platinophilic interactions among mixed-valent tetracyanoplatinate structures. These complexes further illustrate the marked chemical differences and structural diversity of solid-state Np(V) coordination complexes with regard to Np(VI) and U(VI).

An X-ray absorption fine structure spectroscopy study of metal sorption to graphene oxide

Remediation and prevention of environmental contamination by toxic metals is an ongoing issue. Additionally, improving water filtration systems is necessary to prevent toxic metals from circulating through the water supply. Graphene oxide (GO) is a highly sorptive material for a variety of heavy metals under different ionic strength conditions over a wide pH range, making it a promising candidate for use in metal adsorption from contaminated sites or in filtration systems. We present X-ray absorption fine structure (XAFS) spectroscopy results investigating the binding environment of Cd (II), U(VI) and Pb(II) ions onto multi-layered graphene oxide (MLGO). This study shows that the binding environment of each metal onto the MLGO is unique, with different behaviors governing the sorption as a function of pH. For Cd sorption to MLGO, the same mechanism of electrostatic attraction between the MLGO and the Cd⁺² ions surrounded by water molecules prevails over the entire pH range studied. The U(VI), present in solution as the uranyl ion, shows only subtle changes as a function of pH, likely due to the varied speciation of uranium in solution. The adsorption of the U to the MLGO is through a covalent, inner-sphere bond. The only metal from this study where the dominant adsorption mechanism to the MLGO changes with pH is Pb. In this case, under lower pH conditions, Pb is bound onto the MLGO through dominantly outer-sphere, electrostatic adsorption, while under higher pH conditions, the bonding changes to be dominated by inner-sphere, covalent adsorption. Since each of the metals in this study show unique binding properties, it is possible that MLGO could be engineered to effectively adsorb specific metal ions from solution and optimize environmental remediation or filtration for each metal.

Experimental Measurements and Surface Complexation Modeling of U(VI) Adsorption onto Multilayered Graphene Oxide: The Importance of Adsorbate-Adsorbent Ratios

Surface complexation models use experimental adsorption measurements to calculate stability constants that quantify the thermodynamic stability of adsorbed species. However, these constants are often poorly constrained due to nearly complete removal of the solute from solution and/or because the tested adsorbate:adsorbent ratios are not varied sufficiently. Using data sets that quantify the adsorption of U(VI) to multilayered graphene oxide (MLGO), we tested whether three different U(VI):MLGO ratios (3 ppm U; 20-210 mg L^-1 MLGO) affect the ability of nonelectrostatic and diffuse layer models to predict U(VI) adsorption behaviors across a range of ionic strength (1-100 mM) and pH (2-9.5) conditions. Model formulations assumed interactions between discrete MLGO surfaces sites and the most abundant aqueous U(VI) complex(es) within a given pH range. We determined that the observed extents of U(VI) binding require adsorption of more than one U(VI) species (UO₂²⁺ and uranyl hydroxide(s) and/or carbonate(s)) and calculated the respective stability constants for the important U(VI)-MLGO surface complexes. The results also unequivocally illustrated that models using adsorption data from treatments with higher U(VI):MLGO ratios provide better fits throughout the tested range of experimental conditions, meaning that the U(VI)-MLGO stability constants calculated herein can be confidently applied to a range of natural or engineered systems.

Experimental measurements of U24Py nanocluster behavior in aqueous solution

Uranyl peroxide nanoclusters may impact the mobility and partitioning of uranium at contaminated sites and could be used in the isolation of uranium during the reprocessing of nuclear waste. Their behavior in aqueous systems must be better understood to predict the environmental fate of uranyl peroxide nanoclusters and for their use in engineered systems. The aqueous stability of only one uranyl peroxide nanocluster, U60 (K16Li44[UO2(O2)OH]60), has been studied to date [Flynn, S. L., Szymanowski, J. E. S., Gao, Y., Liu, T., Burns, P. C., Fein, J. B.: Experimental measurements of U60 nanocluster stability in aqueous solution. Geochemica et Cosmochimica Acta 156, 94–105 (2015)]. In this study, we measured the aqueous stability of a second uranyl peroxide nanocluster, U24Py (Na30[(UO2)24(O2)24(HP2O7)6(H2P2O7)6]), in batch systems as a function of time, pH, and nanocluster concentration, and then compared the aqueous behavior of U24Py to U60 to determine whether the size and morphology differences result in differences in their aqueous behaviors. Systems containing U24Py nanoclusters took over 30 days to achieve steady-state concentrations of monomeric U, Na, and P, illustrating slower reaction kinetics than parallel U60 systems. Furthermore, U24Py exhibited lower stability in solution than U60, with an average of 72% of the total mass in each nanocluster suspension being associated with the U24Py nanocluster, whereas 97% was associated with the U60 nanocluster in parallel experiments [Flynn, S. L., Szymanowski, J. E. S., Gao, Y., Liu, T., Burns, P. C., Fein, J. B.: Experimental measurements of U60 nanocluster stability in aqueous solution. Geochemica et Cosmochimica Acta 156, 94–105 (2015)]. The measurements from the batch experiments were used to calculate ion activity product (IAP) values for the reaction between the U24Py nanocluster and its constituent monomeric aqueous species. The IAP values, calculated assuming the activity of the U24Py nanocluster is equal to its concentration in solution, exhibit a significantly lower nanocluster concentration dependence than those IAP values calculated assuming an activity of 1 for the nanocluster. The inclusion of a deprotonation reaction for U24Py minimizes the pH dependence of the calculated IAP values. The modeling results suggest that the U24Py nanocluster experiences sequential deprotonation. Taken together, the results indicate that the aqueous behavior of the U24Py nanocluster, like that of U60, is best described as that of an aqueous complex.

Thermal Responsive Ion Selectivity of Uranyl Peroxide Nanocages: An Inorganic Mimic of K(+) Ion Channels

An actinyl peroxide cage cluster, Li48+m K12 (OH)m [UO2 (O2 )(OH)]60 (H2 O)n (m≈20 and n≈310; U60 ), discriminates precisely between Na(+) and K(+) ions when heated to certain temperatures, a most essential feature for K(+) selective filters. The U60 clusters demonstrate several other features in common with K(+) ion channels, including passive transport of K(+) ions, a high flux rate, and the dehydration of U60 and K(+) ions. These qualities make U60 (a pure inorganic cluster) a promising ion channel mimic in an aqueous environment. Laser light scattering (LLS) and isothermal titration calorimetry (ITC) studies revealed that the tailorable ion selectivity of U60 clusters is a result of the thermal responsiveness of the U60 hydration shells.

Hybrid Lanthanide-Actinide Peroxide Cage Clusters

A cage cluster consisting of 31 uranyl and 9 Sm(3+) polyhedra self-assembles in an alkaline aqueous peroxide solution and crystallizes (U31Sm9). Trimers of Sm(3+) polyhedra are templated by μ3-η(2):η(2):η(2)-peroxide groups and link to oxo atoms of uranyl ions. Three such trimers link into a ring through uranyl hexagonal bipyramids, and these are attached through six polyhedra to a unit consisting of 21 uranyl hexagonal bipyramids to complete the cage. Luminescence spectra collected with an excitation wavelength of 420 nm reveal fine structure, which is not observed for a cluster containing only uranyl polyhedra.